Color image forming apparatus

ABSTRACT

Disclosed is a color image forming apparatus in which it is possible to prevent positional deviation in scanning of each optical scanning device without having to improve the lens accuracy to an extreme degree. In a color image forming apparatus of the type which has a plurality of sets of optical scanning devices and image carriers, each of the plurality of optical scanning devices includes a laser unit having a semiconductor laser and a collimator lens, a cylindrical lens formed of a plastic lens material and adapted to effect image formation on the deflection surface of a light deflector from a beam emitted from the laser unit, and an fθ lens system having at least one plastic lens for effecting image formation on the surface of the image carrier from the beam deflected by the light deflector, wherein at least one of the plastic lenses of each of the plurality of optical scanning devices is molded in the same cavity, and wherein at least one of the plastic lenses molded in the same cavity is molded in a multiple cavity mold.

This application is a division of Application No. 09/466,905, filed Dec. 20, 1999, now U.S. Pat. No. 6,304,282, issued on Oct. 16, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a color image forming apparatus and, in particular, to a color image forming apparatus which has a plurality of pairs of optical scanning devices and corresponding image carriers, optical scanning being performed on each image carrier with a beam emitted from each optical scanning device to thereby form an image, or a color image forming apparatus which has an optical scanning device including a multi semiconductor laser and a plurality of image carriers, optical scanning being performed on the image carriers with a plurality of beams emitted from the optical scanning device to thereby form an image. The image forming apparatus is suitably applicable, for example, to a high speed laser printer apparatus, a plurality-drum type color copying machine or a digital color copying machine.

2. Description of the Related Art

Conventionally, in a color image forming apparatus such as a plurality-drum type color printer or a plurality-drum type color copying machine, a laser exposure device, that is, an optical scanning device has been used which provides a plurality of image forming sections corresponding to color components obtained through color separation with image data corresponding to the color components, that is, a plurality of beams.

Generally speaking, an optical scanning device comprises a semiconductor laser device serving as the light source, a first lens unit (collimator lens, cylindrical lens, etc.) for reducing the diameter of a beam emitted from the semiconductor laser device to a predetermined dimension, a light deflector for continuously deflecting and reflecting the beam whose diameter has been reduced by the first lens unit in a direction perpendicular to the direction in which the recording medium is conveyed, a second lens unit (Fθ lens system) for forming an image at a predetermined position of the recording medium from the beam deflected and reflected by the light deflector, etc.

In known examples of such an optical scanning device, a plurality of optical scanning devices are arranged in correspondence with the image forming sections, or a multi-beam optical scanning device capable of providing a plurality of beams is arranged.

In an example of the multi-beam optical scanning device, lens units such as Fθ lenses are arranged in a number corresponding to the number of beams, as disclosed in Japanese Patent Laid-Open No. 3-264970. Further, as disclosed in Japanese Patent Publication No. 2-20986, there is arranged in the vicinity of the photosensitive member (photosensitive drum) a cylindrical lens for causing a plurality of beams to impinge upon a single light deflector and to be condensed by a single fθ lens (scanning lens) to correct any bend of the scanning line.

However, the above conventional multi-beam optical scanning devices have the following problems.

In the device disclosed in Japanese Patent Publication No. 2-20986, the beam impinges upon the fθ lens obliquely, so that the scanning line is curved, and the curved scanning line is corrected by a cylindrical lens. However, the correction cannot be effected completely. When there are a plurality of incident beams, there exist a plurality of incident angles, and the scanning lines with different curvatures cause positional deviation on the surface of the photosensitive member.

In the device disclosed in Japanese Patent Laid-Open No. 3-264970, the number of lens units such as fθ lens arranged corresponds to the number of beams. In the case of this arrangement, the plurality of beams pass the centers of the respective optical systems, so that curving of the scanning line is not easily generated. However, unless the accuracy of the optical systems is improved to an extreme degree, positional deviation in the scanning direction is generated on the surface of the photosensitive member due to errors in production. To avoid this, the lens accuracy is improved to an extreme degree, or a lens position adjusting mechanism is provided, resulting in the apparatus as a whole being complicated. Further, this leads to an increase in production cost.

SUMMARY OF THE INVENTION

It is a first object of the present invention to provide a color image forming apparatus of the type which has a plurality of pairs of optical scanning devices and image carriers, wherein each optical scanning device is appropriately constructed, whereby it is possible to prevent scanning positional deviation of each optical scanning device without having to improve the lens accuracy to an extreme degree.

It is a second object of the present invention to provide a color image forming apparatus of the type which has an optical scanning device including a multi semiconductor laser and a plurality of image carriers, wherein the optical scanning device is appropriately constructed, whereby it is possible to prevent scanning positional deviation without having to improve the lens accuracy to an extreme degree.

In accordance with the present invention, there is provided a color image forming apparatus of the type which has a plurality of sets of optical scanning devices and image carriers, in which beams emitted from the optical scanning devices re led to the surfaces of the corresponding image carriers, scanning being performed on the surfaces of the image carriers with the beams to form images of different colors on the surfaces of the image carriers, a color image being formed from the images formed on the surfaces of the plurality of image carriers,

wherein each of the plurality of optical scanning devices comprises

a laser unit having a semiconductor laser and a collimator lens,

a cylindrical lens consisting of a plastic lens material and adapted to form an image on the deflection surface of a light deflector from a beam emitted from the laser unit, and

an fθ lens system having at least one plastic lens for forming an image on the surface of the image carrier from the beam deflected by the light deflector,

wherein at least one of the plastic lenses of the plurality of optical scanning devices is molded by a multiple cavity mold.

In particular, in each of the plurality of optical scanning devices, the laser unit, the cylindrical lens, and the fθ lens system are held by a single plastic holding member, and the plastic holding member is molded by a multiple cavity mold.

In each of the plurality of optical scanning devices, the light deflector is also held by the plastic holding member.

The cavity combination of the plastic lens and the plastic holding member is specified, and, with another combination, the plastic lens is not attached to the plastic holding member.

The cavity combination of the plastic lens and the plastic holding member is constructed such that it can be recognized from the outside of each optical scanning device.

A plurality of optical scanning devices of the same cavity combination of the plastic lens and the plastic holding member are mounted in the apparatus main body.

The fθ lens system has two plastic lenses, and the cavity combination of the two plastic lenses is specified, no attachment being possible with any other combination.

The cavity combination of the two plastic lenses constituting the fθ lens system is constructed such that it can be recognized from the outside of each optical scanning device.

Optical scanning devices of the same cavity combination of the two plastic lenses constituting the fθ lens system are mounted in the apparatus main body.

The plurality of optical scanning devices shares a light deflector.

Further, in accordance with the present invention, there is provided a color image forming apparatus of the type which has a plurality of sets of optical scanning devices and corresponding image carriers, and in which beams emitted from the optical scanning devices are led to the surfaces of the corresponding image carriers, scanning being performed on the surfaces of the image carriers with the beams to form images of different colors on the surfaces of the image carriers, a color image being formed from the images formed on the surfaces of the plurality of image carriers,

wherein each of the plurality of optical scanning devices has at least one plastic lens and a plastic holding member for holding at least one optical element, and at least one of the plastic lens and the plastic holding member is molded by the same cavity of the multiple cavity mold, and the cavity combination of the plastic lens and the plastic holding member is specified, the plastic lens not being attached to the plastic holding member with any other cavity combination.

In particular, the cavity combination of the plastic lens and the plastic holding member can be recognized from the outside of each optical scanning device.

A plurality of optical scanning devices of the same cavity combination of the plastic lens and the plastic holding member are mounted in the apparatus main body.

Each of the plurality optical scanning devices has a light deflector, which is held by the plastic holding member.

The plurality of optical scanning devices share a light deflector.

Further, in accordance with the present invention, there is provided a color image forming apparatus of the type which has a plurality of optical scanning devices including a multi semiconductor laser and a plurality of image carriers, and in which a plurality of beams emitted from the optical scanning devices are led to the surfaces of the corresponding image carriers, scanning being performed on the surfaces of the image carriers with the beams to form images of different colors on the surfaces of the image carriers, a color image being formed from the images formed on the surfaces of the plurality of image carriers,

wherein the plurality of optical scanning devices comprise

a laser unit having the multi semiconductor laser and a collimator lens,

a cylindrical lens consisting of a plastic lens material and adapted to form images on the deflecting surface of the light deflector from a plurality of beams emitted from the laser unit, and

an fθ lens system having at least one plastic lens for forming images on the surfaces of the image carriers from the plurality of beams deflected by the light deflector, and

wherein at least one of the plastic lenses of the optical scanning device is molded in the same cavity of a multiple cavity mold.

In particular, in the plurality of optical scanning devices, the laser unit, the cylindrical lens, and the fθ lens system are held by a single plastic holding member, and the plastic holding member is molded in the same cavity of a multiple cavity mold.

In the plurality of optical scanning devices, the light deflector is also held by the plastic holding member.

The cavity combination of the plastic lens and the plastic holding member is specified, and the plastic lens does not attach to the plastic holding member with any other combination.

The cavity combination of the plastic lens and the plastic holding member can be recognized from the outside of the optical scanning device.

In the plurality of optical scanning devices, the cavity combination of the plastic lens and the plastic holding member is the same.

The fθ lens system has two plastic lenses, and the cavity combination of the two plastic lenses is specified, the attachment being impossible with any other combination.

The cavity combination of the two plastic lenses constituting the fθ lens system can be recognized from the outside of the optical scanning device.

In the plurality of optical scanning devices, the cavity combination of the two plastic lenses constituting the fθ lens system is the same.

Further, in accordance with the present invention, there is provided a color image forming apparatus of the type which has a plurality of optical scanning devices including a multi semiconductor laser and a plurality of image carriers, and in which a plurality of beams emitted from the optical scanning devices are led to the surfaces of the corresponding image carriers, scanning being performed on the surfaces of the image carriers with the beams to form images of different colors on the surfaces of the image carriers, a color image being formed from the images formed on the surfaces of the plurality of image carriers,

wherein the plurality of optical scanning devices have at least one plastic lens and a plastic holding member holding at least one optical element, wherein at least one of the plastic lens and the plastic holding member is molded in the same cavity of a multiple cavity mold, and wherein the cavity combination of the plastic lens and the plastic holding member is specified, and the plastic lens cannot be attached to the plastic holding member with any other combination.

In particular, the cavity combination of the plastic lens and the plastic holding member can be recognized from the outside of the optical scanning device.

In the plurality of optical scanning devices, the cavity combination of the plastic lens and the plastic holding member is the same.

The plurality of optical scanning devices have a light deflector, which is held by the plastic holding member.

Further, in accordance with the present invention, there is provided a color image forming apparatus of the type which has a plurality of sets of optical scanning devices and image carriers, and in which beams emitted from the optical scanning devices are led to the surfaces of the corresponding image carriers, scanning being performed on the surfaces of the image carriers to form images of different colors on the surfaces of the image carriers, a color image being formed from the images formed on the plurality of image carriers,

wherein each of the plurality of optical scanning devices comprises

a laser unit,

a light deflector for deflecting the beam emitted from the laser unit, and

a lens system having at least one plastic lens for forming an image on a predetermined surface from the beam emitted from the laser unit, and

wherein at least one of the plastic lenses of the plurality of optical scanning devices is molded in the same cavity of a multiple cavity mold.

In particular, in each of the plurality of optical scanning devices, the laser unit and the lens system are held by a single plastic holding member, and the plastic holding member is molded in the same cavity of a multiple cavity mold.

In each of the plurality of optical scanning devices, the light deflector is also held by the plastic holding member.

The laser unit has a multi semiconductor laser emitting a plurality of beams.

Further, in accordance with the present invention, there is provided a color image forming apparatus of the type which has an optical scanning device including a light source portion emitting a plurality of beams and a plurality of image carriers, and in which a plurality of beams emitted from the optical scanning device are led to the surfaces of the corresponding image carriers, scanning being performed on the surfaces of the image carriers with the beams to form images of different colors on the surfaces of the image carriers, a color image being formed from the images formed on the surfaces of the plurality of image carriers,

wherein the optical scanning device comprises

a light source portion,

a light deflector for deflecting a plurality of beams emitted from the light source portion, and

a lens system having at least one plastic lens for effecting image formation on a predetermined surface with the plurality of beams emitted from the light source portion,

wherein there are provided a plurality of said lens systems in correspondence with the plurality of beams, and wherein at least one of the plastic lenses of the lens systems is molded in the same cavity of a multiple cavity mold.

Further, in accordance with the present invention, there is provided a color image forming apparatus of the type which has an optical scanning device including a multi semiconductor laser and a plurality of image carriers, and in which a plurality of beams emitted from the optical scanning device are led to the surfaces of the corresponding image carriers, scanning being performed on the surfaces of the image carriers with the beams to form images of different colors on the surfaces of the image carriers, a color image being formed from the images formed on the surfaces of the plurality of image carriers,

wherein the optical scanning device comprises

a laser unit having a multi semiconductor laser and a collimator lens,

a light deflector for deflecting a plurality of beams emitted from the laser unit,

a cylindrical lens consisting of a plastic lens material and adapted to effect image formation on the deflection surface of the light deflector from the plurality of beams emitted from the laser unit, and

and an fθ lens system having at least one plastic lens for effecting image formation on a predetermined surface from the plurality of beams deflected by the light deflector,

wherein there are provided a plurality of said fθ lens systems in correspondence with the plurality of beams, and wherein at least one of the plastic lenses of the fθ lens systems is formed in the same cavity of a multiple cavity mold.

Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a main part of a color image forming apparatus according to a first embodiment of the present invention;

FIG. 2 is a schematic diagram showing a main part of an optical scanning device according to the first embodiment of the present invention;

FIG. 3 is a diagram illustrating a multiple cavity mold;

FIG. 4 is a schematic diagram illustrating a plastic cylindrical lens according to the first embodiment of the present invention;

FIG. 5 is a schematic diagram illustrating the plastic cylindrical lens of the first embodiment of the present invention;

FIG. 6 is a schematic diagram illustrating the plastic cylindrical lens of the first embodiment of the present invention;

FIG. 7 is a schematic diagram illustrating an fθ lens system according to the first embodiment of the present invention;

FIG. 8 is a schematic diagram illustrating a main part of an optical scanning device according to a second embodiment of the present invention;

FIG. 9 is a schematic diagram illustrating a main part of an optical scanning device according to a third embodiment of the present invention;

FIG. 10 is a schematic diagram illustrating a main part of a color image forming apparatus according to the third embodiment of the present invention;

FIG. 11 is a schematic diagram illustrating a main part of a color image forming apparatus according to a fourth embodiment of the present invention using a multi semiconductor laser;

FIG. 12 is a schematic diagram showing a main part of the optical scanning device shown in FIG. 11; and

FIG. 13 is a schematic diagram showing a main part of the color image forming apparatus of the fourth embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a schematic diagram showing a main part of the color image forming apparatus of the first embodiment of the present invention.

In the drawing, numeral 1 indicates the color image forming apparatus (apparatus main body), numerals 2 a, 2 b, 2 c and 2 d indicate optical scanning devices (hereinafter also referred to as “scanner units”) constructed as described below, and numerals 4 a, 4 b, 4 c and 4 d indicate photosensitive drums (photosensitive members) serving as image carriers. In this embodiment, beams (laser beams) optically modulated based on image information are emitted from the optical scanning devices 2 a, 2 b, 2 c and 2 d, and applied by way of mirrors 3 a, 3 b, 3 c and 3 d to the surfaces of the photosensitive drums 4 a, 4 b, 4 c and 4 d serving as recording media to form latent images. These latent images are formed on the photosensitive drums 4 a, 4 b, 4 c and 4 d uniformly charged by primary chargers (not shown), visualized as cyan, magenta, yellow and black images by developing devices 5 a, 5 b, 5 c and 5 d and sequentially transferred electrostatically by transfer rollers to a transfer material S conveyed by a transfer belt P to thereby form a color image. The mirrors 3 a, 3 b, 3 c and 3 d are constructed so as to adjust the inclination of scanning lines by adjusting mechanisms (not shown).

FIG. 2 is a main part schematic diagram showing one of the optical scanning devices 2 a and the image carrier 4 a corresponding thereto. In the drawing, the components which are the same as those of FIG. 1 are indicated by the same reference numerals.

In the drawing, numeral 6 a indicates a semiconductor laser serving as the light source means. Numeral 7 a indicates a collimator lens which transforms the beam emitted from the light source means 6 a into a substantially parallel beam. Numeral 24 a indicates an opening diaphragm, which restricts the passing beam (quantity of light). The semiconductor laser 6 a, the collimator lens 7 a and the opening diaphragm 24 a are components of the laser unit. Numeral 8 a indicates a cylindrical lens formed of a plastic material (plastic cylindrical lens), which has a predetermined refractive power in a sub-scanning direction perpendicular to the plane of the drawing, and forms in the sub-scanning plane a substantially latent image on a deflection surface 9 a 1 of a light deflector described below from the beam passed through the opening diaphragm 24 a. Numeral 21 a indicates an fθ lens system (scanning lens system) having an fθ characteristic, which is composed of first and second plastic lenses (plastic fθ lenses) 10 a and 11 a.

In the optical scanning device of this embodiment, the beam emitted from the semiconductor laser 6 a is transformed into a substantially parallel beam by the collimator lens 7 a, and the beam (quantity of light) is restricted by the opening diaphragm 24 a before the beam impinges upon the plastic cylindrical lens 8 a. In the main scanning section, the substantially parallel beam impinging upon the plastic cylindrical lens 8 a is output as it is. In the sub-scanning section, it is converged and forms a substantially latent image (which is longitudinal with respect to the main scanning direction) on the deflection surface 9 a 1 of the light deflector 9 a. The beam deflected by the deflection surface 9 a 1 of the light deflector 9 a is passed through the fθ lens system 21 a, whereby its scanning linearity is corrected, and is led to the surface of the photosensitive drum 4 a through the mirror 3 a. By rotating the light deflector 9 a in the direction of the arrow A, optical scanning is performed in the direction of the arrow B on the surface of the photosensitive drum 4 a. Then, as described above, latent images of C (cyan), M (magenta), Y (yellow) and B (black) are formed on the surfaces of the corresponding photosensitive drums. After this, they are multi-transferred to the transfer material to form a single color image.

As shown in FIG. 3, in this embodiment, at least one of the plastic lenses forming the optical scanning devices is molded in the same cavity, and at least one of the plastic lenses molded in the same cavity is molded in a multiple cavity mold (22K, 8K, 10K, 11K).

Further, in this embodiment, in each optical scanning device, the laser unit, the cylindrical lens, and the fθ lens system are held by a single plastic holding member, and the plastic holding member is molded in the same cavity, and the plastic holding member molded in the same cavity is molded in a multiple cavity mold. Further, the light deflector of each optical scanning device is held by the plastic holding member.

In this embodiment, the plastic holding member 22 a of the scanner unit 2 a, the plastic cylindrical lens 8 a, and the first and second plastic fθ lenses 10 a and 11 a are molded in a plurality of cavities, and the cavity combination is specified, the attachment of the plastic lens to the plastic holding member 22 a being impossible with any other combination. For example, as shown in FIG. 3, the plastic holding members 22 are molded in two cavities, the cylinders 8 are molded in six cavities, the plastic lenses 10 are molded in four cavities, and the plastic lenses 11 are molded in three cavities.

In this embodiment, the molded products used are molded in two cavities A1 and A2. The number of cavities, which is determined depending upon the molding tact, is not particularly restricted. Naturally, in some cases, only one cavity is used. In FIG. 3, numerals 22K, 8K, 10K, and 11K indicate multiple cavity molds.

As shown, for example, in Table 1, the cavity combinations are specified, no attachment being possible with any other combination.

TABLE 1 Holding member Plastic Plastic Scanner box 22 Cylinder 8 lens 10 lens 11 Combination 1 A1 A1 A2 A1 Combination 2 A2 A1 A1 A1

The above combinations 1 and 2 are determined such that the aberration such as scanning line curvature and field curvature is improved in terms of tolerance. A plurality of scanner units of the same combination are mounted on a single apparatus.

FIGS. 4, 5 and 6 are diagrams illustrating the plastic cylindrical lens 8 a shown in FIG. 2. In FIG. 4, numeral 12 indicates a positioning member for the positioning on a plastic scanner box 2 a′, and numeral 13 indicates a cavity determining dowel. The cavity determining dowel 13 differs depending on the cavity of the plastic scanner box 2 a′ (plastic holding member 22 a), and only a plastic cylindrical lens 8 a of a specified cavity can be attached. FIG. 5 is a diagram illustrating the plastic cylindrical lens of another cavity; it differs in the configuration of the portion in the vicinity of the cavity determining dowel 13. FIG. 6 is a perspective view of the plastic cylindrical lens 8 a.

FIG. 7 is a diagram illustrating the first and second plastic fθ lenses 10 a and 11 a constituting the fθ lens system 21 a. In the drawing, numeral 14 indicates pins for positioning the first and second plastic fθ lenses 10 a and 11 a, which are above the plastic scanner box 2 a′, in the Y-direction, and numeral 15 indicates pins for positioning them in the X-direction. Numeral 16 indicates cavity determining dowels, and numeral 17 indicates pins indicating the cavity of the first and second plastic fθ lenses 10 a and 11 a. Since the pins 17 indicating the cavities differ in position depending upon the lens cavity, the cavity combination of the plastic scanner box 2 a′ (plastic holding member 22 a) and the first and second plastic fθ lenses 10 a and 11 a is determined according to the cavity determining dowel 16.

In this embodiment, the color image forming apparatus is constructed as described above, whereby it is possible to prevent positional deviation in scanning of each optical scanning device without having to improve the lens accuracy to an extreme degree.

FIG. 8 is a schematic diagram showing a main part of an optical scanning device according a second embodiment of the present invention. In the drawing, the components which are the same as those shown in FIG. 2 are indicated by the same reference numerals.

This embodiment differs from the above-described first embodiment in that only the cavity combination of the first and second plastic fθ lenses is determined, no attachment being possible with any other combination, and that the cavity combination can be recognized from the outward appearance of the plastic scanner box 2 a′ (optical scanning device 2 a). Apart from the above, the construction and the optical effect of this embodiment are substantially the same as those of the first embodiment described above, and the same effect can be thereby achieved.

That is, in the drawing, numeral 17 indicates pins determining the cavity combination of the first and second plastic fθ lenses 10 a and 11 a, any other cavity combination being impossible. In this embodiment, there are two cavities A1 and A2. Numeral 18 indicates a pin indicating the A1 cavity of the second plastic fθ lens 11 a, which passes through the plastic scanner box 2 a′ in the Z-direction. The A2 cavity is equipped with no pin 17. The cavity combination of the first and second plastic fθ lenses 10 a and 11 a can be recognized according to whether the pin 17 passes through the plastic scanner box 2 a′ or not.

In this way, in this embodiment, only the cavity combination of the first and second fθ lenses is determined, and the cavity combination can be recognized from the outward appearance of the plastic scanner box 2 a′, whereby combination error in assembly and replacement is prevented. Further, scanner units of the same combination are mounted on the apparatus main body.

While in this embodiment four lenses of the same combination are incorporated, it is possible to incorporate lenses of any combination as long as the combination of the first and second plastic fθ lenses is determined without involving any problem in performance.

Further, while in this embodiment the cavity combination is recognized according to whether there is a pin or not, this should not construed restrictively. It is also possible, for example, to identify the cavity combination by attaching a label or the like to the plastic scanner box at the time of assembly. It is possible to adopt a construction in which the cavity combination of the plastic lens and the plastic holding member can be recognized from the outside of each scanner unit.

FIG. 9 is a schematic diagram showing a main part of the third embodiment of the present invention. In the drawing, the components which are the same as those shown in FIGS. 1 and 2 are indicated by the same reference numerals.

This embodiment differs from the first embodiment described above in that in each optical scanning device, the laser unit, the plastic cylindrical lens, and the fθ lens system are held by a single plastic holding member and that the light deflector is shared. As in the first embodiment described above, this plastic holding member is molded in the same cavity and in a multiple cavity mold.

That is, in the drawing, numeral 23 a indicates a plastic holding member, which holds a semiconductor laser 6 a, a collimator lens 7 a, a plastic cylindrical lens 8 a, and first and second plastic fθ lenses 10 a and 11 a. As in the first embodiment, the cavity combination of the plastic holding member 23 a, the plastic cylindrical lens 8 a, and the first and second plastic fθ lenses 10 a and 11 a is determined.

In this embodiment, a plastic holding member 23 a′ which is the same as the plastic holding member 23 a is arranged symmetrically on the plastic scanner box 2 a′ with the polygon mirror 9 a′ therebetween, whereby the polygon mirror 9 a′ is shared. That is, one plastic holding member 23 a′ is reversed with respect to the plane of the drawing.

FIG. 10 is a schematic diagram showing the construction of the entire apparatus of this embodiment. As shown in the drawing, in this embodiment, two scanner units 2 a″ and 2 b″ in which two sets of optical scanning devices are provided are mounted.

By thus constructing the color image forming apparatus, this embodiment provides the same effect as the above-described embodiments.

While in each embodiment a semiconductor laser emitting a single beams is used as the light source means, this should not be construed restrictively. For example, it is also possible, as shown in FIGS. 11 and 12, to form the light source means forming the optical scanning devices 32 and 32′ by multi semiconductor lasers. The present invention is also applicable as in the above embodiments when a plurality of beams are emitted from the optical scanning device 32 (while two beams are emitted from each optical scanning device in FIGS. 11 and 12, it is also possible to emit two or more beams) and led to the surface of the photosensitive drum 33 a (33 b) to form a color image from the images formed on the plurality of photosensitive drums 33 a and 33 b.

FIG. 13 is a schematic diagram showing a main part of a color image forming apparatus of the fifth embodiment of the present invention using a multi semiconductor laser.

A rotary polygon mirror 41 is formed by two polygon mirror portions 41 a and 41 b and an intermediate section connecting these polygon mirror portions 41 a and 41 b. These are formed by machining a single metal block. A laser light source 40 is provided which emits two beams so that they impinge upon the polygon mirror portions 41 a and 41 b. Cylindrical lenses 45 a and 45 b are provided between the laser light source 40 and the polygon mirror portions 41 a and 41 b. In the emitting direction of the polygon mirror portions 41 a and 41 b, there are provided upper and lower scanning lenses 46 a, 47 a, 46 b and 47 b. Further, in the emitting direction of the upper scanning lenses 46 a and 47 b, there is provide an image carrier 48 a, and, in the emitting direction of the lower scanning lenses 46 b and 47 b, there is provided a reflection mirror 49, and, in the reflecting direction of the reflection mirror 19, there is provided an image carrier 48 b.

Due to this construction, the two laser beams which are emitted from the laser light source 40 and which are parallel beams are caused to focus on the upper polygon mirror portion 41 a and the lower polygon mirror portion 41 b of the rotary polygon mirror 41 by the cylindrical lenses 43 a and 43 b having refractive power only in the sub-scanning direction. Further, the beams deflected by the rotary polygon mirror 41 are transmitted through the scanning lenses 46 a, 47 a, 46 b and 47 b having an fθ characteristic, and the upper laser beam directly led to the image carrier 48 a, and the lower laser beam is reflected by the reflection mirror 49 and led to the image carrier 48 b to perform optical scanning.

Here, the laser light source 40 is a laser unit having a multi semiconductor laser and a collimator lens, and parallel beams L1 and L2 are emitted from the laser light source 40.

Further, the laser light source 40, the rotary polygon mirror 41, the cylindrical lens 45 a, the scanning lenses 46 a and 47 a, the cylindrical lens 45 b, and the scanning lenses 46 b and 47 b are accommodated in a plastic holding member 50.

Here, the first lens system which effects image formation on the image carrier 48 a from the first beam L1 emitted from the laser light source 40 is formed by the cylindrical lens 45 a and the scanning lenses 46 a and 47 a, at least one of which is a plastic lens.

Further, the second lens system which effects image formation on the image carrier 48 b from the second beam. L2 emitted from the laser light source 40 is formed by the cylindrical lens 45 b, and the scanning lenses 46 b and 47 b, at least one of which is a plastic lens.

At least one of the plastic lenses of each lens system is a lens molded in the same cavity of a multiple cavity mold.

As described above, in accordance with the present invention, it is possible to realize a color image forming apparatus of the type which has a plurality of sets of optical scanning devices and image carriers, wherein, by appropriately constructing each optical scanning device, it is possible to prevent positional deviation in scanning of each optical scanning device without having to improve the lens accuracy to an extreme degree.

Further, as described above, in accordance with the present invention, it is possible to realize a color image forming apparatus of the type which has an optical scanning device including a multi semiconductor laser and a plurality of image carriers, wherein, by appropriately constructing the optical scanning device, it is possible to prevent positional deviation in scanning of the optical scanning device without having to improve the lens accuracy to an extreme degree.

While the present invention has been described with reference to what are presently considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 

What is claimed is:
 1. A scanner unit comprising: a laser unit; a lens system having at least one plastic lens adapted to effect image formation on a predetermined surface from the beam emitted from said laser unit; and a plurality of optical scanning devices, each of which includes said laser unit and said lens system, wherein at least one of the plastic lenses of a same optical property of each of said plurality of optical scanning devices is molded in a same cavity of a multiple cavity mold.
 2. A scanner unit according to claim 1, wherein each of said plurality of optical scanning devices has a light deflector.
 3. A scanner unit according to claim 1, wherein said plurality of optical scanning devices share a light deflector.
 4. A scanner unit according to claim 1, wherein said laser unit has a multi-laser emitting a plurality of beams.
 5. A scanner unit according to claim 1, wherein all of the plastic lenses of a same optical property of each of said plurality of optical scanning devices is molded in a same cavity of a multiple cavity mold. 